hotfix(vpn): fix vpn stop when down command is empty

internal/vpn/cleanup.go

@@ -13,11 +13,13 @@ func (l *Loop) cleanup() {
 	settings := l.GetSettings()
 
 	var err error
+	if *settings.DownCommand != "" {
 		commandString := strings.ReplaceAll(*settings.DownCommand, "{{VPN_INTERFACE}}", getVPNInterface(settings))
 		err = l.cmder.RunAndLog(context.Background(), commandString, l.logger)
 		if err != nil {
 			l.logger.Error("failed to run VPN down command: " + err.Error())
 		}
+	}
 
 	for _, vpnPort := range l.vpnInputPorts {
 		err := l.fw.RemoveAllowedPort(context.Background(), vpnPort)